Lyndon Jones is currently a tenured Associate Professor at the School of Optometry and Associate Director of the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada. He graduated in Optometry from the University of Wales, UK in 1985 and gained his PhD from the Biomaterials Research Unit at Aston University, UK in 1998. He is a Fellow and Diplomate of the American Academy of Optometry, has authored over 200 papers and conference abstracts, one text-book and given over 200 invited lectures at conferences worldwide.

Part 1

Introduction

Silicone hydrogel contact lenses first appeared commercially in 1998 and since then have shown tremendous growth, with approximate sales in 2003 of $150 million. Initially developed for the extended wear market, practitioners have embraced the use of such materials for both overnight and daily wear use.

The purpose of this editorial is to briefly review the development of contact lens materials, in particular the development of silicone hydrogels, and to update practitioners on recent developments in this area. More detailed reviews on these issues can be obtained from several other sources (1-11).

Conventional Hydrogel Materials

Conventional hydrogel materials are polymers that are typically composed of several monomers joined together in chains which are linked together at intervals by small amounts (usually <1%) of cross-linking agents to form a polymer network. The commonest, and in some ways the simplest, of these is the first hydrogel material used for contact lens wear, poly(2-hydroxyethyl methacrylate) or polyHEMA (sometimes simply referred to as HEMA, the constituent monomer), which was developed by Wichterle in the 1960’s (12). This is a so-called “homopolymer” because it contains only one type of monomer unit and comprises many units of hydroxyethyl methacrylate joined by a cross-linker (EGDMA). PolyHEMA is easily fabricated into contact lenses, is relatively cheap to produce, highly flexible, dimensionally stable to changes in pH and temperature and has proved to be a very successful contact lens material. The principal disadvantage of polyHEMA is that it relies upon water to transport oxygen across the material and water has a limited ability to dissolve and transport oxygen, with an approximate oxygen permeability (Dk) of around 80 Dk units. From a clinical perspective, oxygen transport to the cornea depends upon both permeability of the material (Dk) and also thickness (t), with thinner lenses providing the cornea with more oxygen. The term (Dk/t) describes the oxygen transmissibility of a lens and gives a quantitative indication of the amount of oxygen that a lens-wearing eye will receive through the lens and is a more clinically useful number than Dk, which gives no indication of the effect of lens thickness or lens design of individual lenses.

In order to increase the Dk of a conventional hydrogel contact lens material beyond that of polyHEMA, it is necessary to incorporate monomers that will bind more water into the polymer. These higher water content materials typically use HEMA or methyl methacrylate (MMA) in conjunction with more hydrophilic monomers such as N-vinyl pyrrolidone (NVP) or methacrylic acid (MA). The constituent monomers used determine the various physical and chemical properties of the material, with MA containing materials having a significant degree of negative surface charge. Table 1 details the monomers used in many common contact lens materials, along with their water content, FDA grouping and their registered United States Adopted Name (USAN).

Silicone Hydrogel Materials

Silicone-rubber based flexible contact lenses are not new, with silicone-elastomeric lenses being used for therapeutic and paediatric applications for many years ( 13 ). These lenses offer exceptional oxygen transmission and durability, but a number of major limitations are associated with their use in clinical practice. Fluid is unable to flow through these lens materials, resulting in frequent lens binding to the ocular surface ( 14 ), and the lens surfaces are extremely hydrophobic, resulting in marked lipid deposition ( 15 ). In silicone-hydrogel materials, silicone rubber is combined with conventional hydrogel monomers. The silicone component of these lens materials provides extremely high oxygen permeability, while the hydrogel component facilitates flexibility, wettability and fluid transport, which aids lens movement. The process of combining conventional hydrogel monomers with silicone proved to be an enormous challenge and it has taken over 20 years of considerable intellectual input and financial resources for these materials and designs to be created. Indeed, the process of combining these monomers has been likened to efforts of combining oil with water, while maintaining optical clarity ( 1 ).

Three silicone hydrogel lens materials are currently commercially available, with their major features being summarised in Table 2. A major problem with the development of silicone hydrogels relates to the fact that successful contact lens materials must permit the transmission of not only oxygen but also ions. One approach that may be used to achieve this goal involves the incorporation of two “phases” into the materials. Phase separation occurs when very chemically dissimilar molecules (such as oil and water) coexist within a material. This approach to contact lens material development was historically avoided because it usually results in an opaque material. However, techniques have been developed in which the phase separation is controlled, such that the phase size is far smaller than the wavelength of light, resulting in optically clear materials ( 7, 16 ).

CIBA Vision’s Focus Night & Day material, lotrafilcon A, employs such a co-continuous biphasic or two channel molecular structure, in which the phases persist from the front to the back surface of the lens ( 7 ). The siloxy phase facilitates the solubility and transmission of oxygen and the hydrogel phase transmits water and oxygen, allowing good lens movement. The two phases work concurrently, to allow the co-continuous transmission of oxygen and aqueous salts. Lotrafilcon A is comprised of a fluoroether macromer co-polymerised with the monomer tris (trimethyl-siloxy)- gamma -methacryloxy-propylsilane (TRIS - used in the preparation of RGP materials) and N,N-dimethyl acrylamide (DMA), in the presence of a diluent ( 6 ). The resultant silicone hydrogel material has a water content of 24% and an oxygen permeability (Dk) of 140 barrers ( 17 ).

Vistakon’s Acuvue Advance material, galyfilcon A, is the newest of the three materials and very little has been published to-date on the material composition ( 19 ), although some deductions can be made from the patent literature dealing with Vistakon’s “HydraClear™” technology. It has a higher water content than the other two materials (47%) and thus the lowest Dk (60 barrers). Whereas both PureVision and Focus Night & Day are FDA approved for overnight use, Acuvue Advance is only approved for daily wear. It is the first of the so-called “second generation” silicone hydrogels ( 4 ) and is the only one available thus far that has an inversion marker and UV blocker, with a reported Class 1 UV protection, blocking >90% of UVA and >99% of UVB rays ( 19 ).

The differences between silicone hydrogel lens materials and conventional hydrogels are considerable, and may be broadly divided into differences between the bulk properties and those attributable to the surface. Part 2 of this review will investigate these differences and demonstrate why silicone hydrogels materials are so unique.